I am not an expert in licensing legalese, but that strikes me as awfully wrong. If I am e.g. a musician and I license my work to be used by B for specific purposes and B unilaterally signs an agreement with A that gives them rights to my stuff, the question is gonna be what MY licensing terms allow. They have an agreement with me. Whether they enter other agreements is their decision to make, but they still have to fulfill my terms.

Sure, they should not sign things that put them into the spot of having to break my licensing agreement with me, but the fact that these kind of legal stretches are even seen as valid in the US reassures me in my stance to avoid business relations with the US. I prefer countries with a legal system that can't easily be bent out of shape using money.

You really hit the nail on the head. I can't sign over rights to stuff I don't own. Is there a good chance that this move could blow up in Meta's face, in which the Party B's organize (or are ordered) en masse not sign the license agreement and leave Zuck without the legal ability to steal while also losing marketshare?

I think more and more people like you around the world are realizing not only how corrupt the U.S. is, but that it's power and influence are in a sharp state of decline and perhaps not worth the risk at this point.